1. Field of the Invention
The present invention relates generally to ink-jet writing instruments and, more particularly, to an ink-jet system having a pen and a detachable ink reservoir in which the pen includes a mechanism for preventing nozzle drool and air ingestion nozzle depriming.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 (PRIOR ART) depicts an ink-jet hard copy apparatus, in this exemplary embodiment, a computer peripheral, color printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller (usually a microprocessor or application specific integrated circuit ("ASIC") controlled printed circuit board, not shown) connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC's. Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station where graphical images or alphanumeric text is created using state of the art dot matrix manipulation techniques. A carriage 109, mounted on a slider 111, scans the print medium. An encoder strip 113 and appurtenant devices are provided for keeping back of the position of the carriage 109 at any given time. A set 115 of individual ink-jet pens, or print cartridges, 117A-117D are releasably mounted in the carriage 109 for easy access (generally, in a full color system, inks for the subtractive primary colors, cyan, yellow, magenta (CYM) and true black (K) are provided). Each pen or cartridge has one or more printhead mechanisms (not seen in this perspective) for "jetting" minute droplets of ink to form dots on adjacently positioned print media. Once a printed page is completed, the print medium is ejected onto an output tray 119. If the set 115 of inking units are reusable pens, one or more off-axis ink reservoirs 121 are provided, including fluidic coupling mechanisms 123 between the reservoirs 121 and the individual pens 117.
Print cartridges are generally fully self-contained inking units intended for one-time use and replacement. Ink-jet pens are inking units which separate semipermanent printhead mechanisms from the ink supply either by having an ink reservoir off-axis from the pen coupled thereto by appropriate fluidic linkage, or a separate, snap-on or press-fit, replaceable, ink supply for each pen. Pens tend to be constructed to use free-ink or other equivalent colorant, toner, or the like, in a contained but unencumbered liquid form rather than in a saturated material (such as polyurethane foam used in some print cartridges) to facilitate the repeated ink supply replacements. The printheads in both cartridges and pens generally require a mechanism to prevent the free flow of ink through the nozzle orifices when the printhead is not activated. Without such control, ink may leak, or "drool," onto the printing surface or into the printer mechanism. Such leaking ink may also build up and cake on the printhead itself, impairing proper operation. Complex pen service stations are often provided as part of the hard copy apparatus where printheads can be wiped or activated to "spit" away excess ink. Moreover, if a proper nozzle pressure balance is not maintained, a printhead can ingest air and "deprime" the nozzles. Complex priming pumps are provided as part of the hard copy apparatus in systems where depriming has been found to be problematic.
To alleviate this problem more directly, many ink-jet printers supply ink from the reservoir to the printhead at a slight under pressure (also referred to in the art as "back-pressure" or "negative pressure" operation), lower than the ambient atmospheric pressure at the printhead. To be effective, this pen back-pressure must be maintained consistently and predictably within a desired operating range. That is, the pen back-pressure must be large enough to prevent the unwanted free flow of ink through the orifices when the pen is not in use, yet at the same time small enough so that the printhead, when activated, can overcome the back-pressure and eject ink droplets in a consistent and predictable manner. This back-pressure will be affected by changes in either or both the ambient atmospheric and the internal pressure conditions. Likewise, temperature variations may cause the ink and air within the ink-jet pen to contract or expand, also affecting the back-pressure. Depending on the exact changes experienced, without such compensation, ink will either drool from the nozzles or air will be ingested through the nozzles. Therefore, these factors must be accounted for and a mechanism incorporated to maintain the back-pressure within the predetermined, desirable operating range.
In a foam reservoir print cartridge, the capillary action of the ink-soaked foam will generally be sufficient to create the desired back-pressure. In a free-ink reservoir type ink-jet pen, a variable volume, on-board, ink containment supply is often employed. As examples: the reservoir may be of a biased, flexible material which can expand or contract; an ink containment chamber may be provided which includes an internal pressure regulating device; a spring pulls an ink-filled bladder membrane outwardly to create a slight negative pressure inside the ink reservoir, a check valve in a printing device with an on-board ink reservoir that maintains a constant pressure difference between the ink reservoir and the ink-jet printhead; spring-loaded ink bag type of pressure regulated ink cartridge; diaphragm type pressure regulator located on-board an ink-jet pen using an off-board ink reservoir; or diaphragm and other atmospheric pressure controlled type mechanism pressure regulators located on-board an ink-jet pen using an off-board ink reservoir.
Back-pressure needs to be controlled within a specified tolerance limits so that the printhead can print properly. Print quality fluctuations are directly related to back-pressure fluctuations. Too little back-pressure can lead to poor print quality and ink leakage; too much back-pressure can starve the printhead which will also affect print quality and printhead life since running an ink-jet pen dry can damage the printhead mechanism. The back-pressure needs to be maintained regardless of the printing conditions, but in the prior art has fluctuated as a function of ink level in the on-axis supply (where on-axis designates a mechanism that travels with the carriage 109 (FIG. 1) during scanning) or as a function of the ink flow rate from an off-axis reservoir. In other words, a delicate balance must be maintained to prevent drooling from or depriming of the printhead nozzles.
One of the remaining technical challenges of such pen systems is the managing of ink and air remaining in the pen and printhead unit when the ink supply is decoupled. Without some means for controlling vacuum in the pen when the ink supply is removed, ink will drool from the nozzles or air will be ingested through the nozzles resulting in a deprimed condition. As consumer pricing competition increases, there is a need for simple, inexpensive systems that solve theses problems.